Automatic noise suppressor circuit



Feb. 9, 1937. o. H. SCHADE AUTOMATIC NOISE SUPPRESSOR CIRCUIT Filed Aug.10, 1935 v mwm,

Patented Feb. 9, 1937 UNITED STATES PATENT OFFICE Otto H. Schade, WestCaldwell, N. J., assigner to Radio Corporation of America, a corporationof Delaware Application August 10, 1935, Serial No. 35,602

8 Claims.

My present invention relates to automatic control circuits for radioreceivers, and more particularly to a novel and improved type ofbackground noise suppressor for a radio receiver which utilizesautomatic gain control.

One of the principal operating defects of most background noisesuppressor circuits of radio receivers equipped with automatic gaincontrol networks is the existence of a range of transi- 10 tion fromcut-oi bias to normal bias on the noisecontrolled tube. This transitionrange causes, when the control is applied to the audio system, highdistortion of the audio frequency signal. It has been found that wherethe background noise i5 suppressor circuit possesses a trigger actionthen the transition range effect is eliminated. The trigger actionusually involves a network which is capable of sluiting the bias valueof the controlled tube from cut-off to normal without any other possiblestable value.

One of the main objects of my present invention may be said to reside inthe provision of a background noise suppressor circuit for a radioreceiver of the AVC type, which suppressor circuit possesses a triggeraction characteristic in that it is capable of applying a cut-oi bias toan audio amplier without the necessity of adjusting the bias through anytransitional bias value, and the suppressor circuit essentiallycomprising an oscillator under the control of the incoming signal, andwhose oscillations are employed to produce the control bias for theaudio amplifier to be controlled.

Another important object of the invention is to provide in a radioreceiver equipped with an automatic volume control arrangement, a noisesuppressor system which functions to regulate the operating bias of atleast one audio amplifier of the receiver, and the suppressor circuitincluding an electron discharge tube which comprises an oscillatorsection and a pair of rectifier sections, one of the rectifier sectionsfunctioning to control the operation of the oscillator section, and theother rectifier section functioning to rectify the oscillatory output ofthe oscillator section for the purpose of producing the control bias forthe audio tube which is to be controlled.

Another object of the present invention is to provide a superheterodynereceiver of the type including an automatic volume control system whosefunction it is tc maintain the signal amplitude at the demodulatorsubstantially uniform over a wide range of signal amplitude variation atthe signal collector, the receiver being particu- (Cl. Z50-20) larlycharacterized by its inclusion of a background noise suppressorarrangement, whose operation is dependent upon the decrease of thereceived signal amplitude below a predetermined amplitude, thesuppressor arrangement compris- 5v ing an oscillator, whose oscillationsare rectied, and the direct current voltage component of the rectifiedoscillations being used to render the reproduction of the demodulatedsignals ineflicient.

Still other objects of the invention are to im- 10 prove generally theoperating efficiency of superheterodyne receivers equipped with noisesuppressor arrangements, and more especially to provide back-groundnoise suppressor circuits for radio receivers equipped with automaticvolume l5 control, which noise suppressor circuits are not only reliableand efficient in operation but economically manufactured and'assembledin radio receivers.

The novel features which I believe to be char- 20 acteristic of myinvention are set forth in particularity in the appended claims; theinvention itself, however, as to both its organization and method ofoperation will best be understood by reference to the followingdescription taken vin 25 connection with the drawing in which I haveindicated'diagrammatically a circuit organization whereby my inventionmay be carried into effect.

In the drawing: 30

Fig. 1 shows a circuit diagram of a superheterodyne receiver embodyingthe invention,

Fig. 2 graphically illustrates the operation of the noise suppressornetwork in the circuit of Fig. 1. A35

Referring now to the circuit diagram in Fig. 1, it is pointed out thatthis circuit is that of a superheterodyne receiver; the receivercomprising the usual signal collector A which may be of the groundedantenna type. The collector may, 40 if desired, be the usual automobileradio signal collector, a loop antenna, and even a radio frequencydistribution line of a hotel or apartment house. Regardless of thenature of the signal collector, the collected signals are impressed upon45 a tunable radio frequency amplifier I which may ybe of the pentodetype, and utilize a tube of the 6D6 type. The input circuit of theamplifier is shown as comprising a single tunable circuit 2 which isprovided with a variable tuning con- 5o denser 3. Of course, thoseskilled in the art are fully aware of the fact that the selector networkpreceding amplifier l may be of the band pass type wherein the band passnetwork comprises a pair of tunable circuits having over-optimum 55known as a pentagrid converter tube, commercially designated by thesymbol 6A7. Since this tube and its circuits are Ywell known. to thoseskilled in the -art at the present time, it is believed sufficient topoint out that theinput signal for the converter circuit is impressedupon the tunable input circuit 4, while the circuit 6 functions as thelocal oscillator circuit. The variable tuning condenser 'I in the signalinput circuit 4 is arranged to have its rotors mechanicallyunicontrolled with the rotors of condenser 3. The variable tuningcondenser 8 of the oscillator circuit 6 also has its rotorsuni-controlled with the rotors of condensers 3 and 1, and the dottedlines 9 represent the, mechanical uni-control mechanism employed to varythese tuning condensers in unison.

Assuming that the operating intermediate frequency of the system ischosen to be 456 k. c., although it is to be understood that any othervalue from 175 k. c. to a value of the order of 500 k. c. may beselected, then it will be clear that the rangeof the local oscillatorcircuit 6 will be from 1006 k. c. to 1956 k. c. The intermediatefrequency energy is taken out of the converter network by means of theresonant circuit I Il which tuned by the fixed condenser includedtherein to the 456 k..c. I. F. The I. F. energy in circuit I0 may beamplified in one or more stages of I. F: amplification.

The I. F. amplifier II may be a tube of the 6D6 type, which is apentode, and the resonant input circuit I 2 thereof is xedly tuned tothe operating I. F. of 456 k. c. Circuits VII) and I2 are coupled tonormally provide a sharp tuning single peaked characteristic. vThis isobtained by coupling the coils of the two circuits with slightly lessthan optimum coupling. In order to provide a means for manuallyadjusting the width of the I. F. band transmitted to the demodulator ofthe Y receiver, there is provided an auxiliary adjustable capacity ineach of circuits I0 and I2. Each of these auxiliary capacities isadjustable over a small'range of frequencies, and the condensers aredesigned to vary the I. F. frequency in opposite senses.

Thus, when it is desired to widen the I. F. band width, the auxiliarycapacities are adjusted so that the frequency of circuit I0, forexample,

will Vary a predetermined frequency value below the I. F., and thefrequency of circuit I2 will be varied to the same frequency value abovethe I. F. 'I'his band widthl control is provided in order to accommodatethe receiver to local or Weak station reception ;k when receiving localstations high quality is desired and all the side band frequencies of`the carrier are thus included. Since the auxiliary capacities are largein size, the adjustment thereof is made by a step switch.

Itis to be clearly understood that the band width control mechanismshown associated with ,circuits I0.and I2 may be replaced by any other'type of band width control.

In superheterodyne receivers of the so-ca-lled high fidelity type such aband width control is usually included because it is desired to have theoperator select at his convenience the type of selectivity he Wishes t0employ, and which selectivity is dependent upon the range of side bandfrequencies he desires tov have reproduced. The network II is followedby a network I3 which includes a tube of the 6B7 type. This type of tubecomprises a pentode section and a diode section, the pentode being usedfor intermediate frequency amplification as in the case of network I I.

It is to be clearly understood that there may be employed between theoutput of tube II and the input of the pentode section of the following6B7 tube a network exactly corresponding to that shown between networks5 and II. The diode section of network I3 is employed in this case forAVC rectification. That is the I. F. energy may be impressed upon thediode section and rectified, and the direct current component ofrectified signals is then employed as AVC bias in the Well known manner.

The pentode sectionof network I3 will, therefore, function as an I. F.amplifier, and, if desired, it may also function as an I. F. amplifierfor the energy prior `to impression on the diode section of network I3.Reference is made to my applica.- tion Serial No. 673,389, led May 29,1933, Patent No. 2,039,666, granted May 5, 1936 which shows such anarrangement wherein the AVC diode rectifier derives its I. F. energy inamplified form from the output of the pentode section of a 6B7 I. F.amplifier tube. It is to be clearly understood that the specificconstruction of the AVC network is not an important element of thepresent invention.

What is essential is that there be provided in the receiver an AVCarrangement which functions in the well known manner to decrease thegain of each control tube as the received signal amplitude rises above apredetermined carrier amplitude. In Vthis way the signal amplitude atsection thereof has its strapped anodes connected 2 to the highalternating potential side of input circuit I5, whereas the cathode ofthe tube is connected to the low alternating potential side through theload resistor II` which is properly by-passed for I. F. currents bycondenser I8. The l input circuit I5 which feeds the demodulator, or.second detector, is tuned to the operating I. F., and the audiocomponent of demodulated signal energy is impressed upon the signal gridof the pentode; section of tube I6 through a path which includes thevariable tap 20 and the condenser I9.

The amplified audio component of demodulated signal energy is thentransmitted through the audio frequency transformer 2I to the followingaudio network. 'I'his subsequent audio network may comprise any of theWell known types of audio networks well known to those skilled in theart. For example, this following network may comprise a system of theytype disclosed by me in Fig. 8 of my application Serial No. 698,407,

fifi

erodyne receiver wherein the signal amplitude at the second detectorinput circuit I5 is maintained substantially uniform regardless offading effects. As is well known to those skilled in the art backgroundnoise reproduction increases in a most annoying fashion when tuning areceiver, as shown in Fig. l, between signal channels. Such noisereproduction also increases to a great extent whenever the signalamplitude received decreases below the operating minimum level.Background noise suppressor arrangements have been utilized in the pastto automatically quiet, or mute, the receiver whenever the AVC system isnot functioning to reduce the gain of the controlled stages, and thereceived signal amplitude drops below a predetermined intensity level.The principal defect of most of these noise suppression circuits hasbeen in the existence of a range of transition Ifrom cut-off bias tonormal bias on the controlled tube.

W'here the noise control bias is applied to the audio stage such atransition range causes high distortion of the audio signal. The mostdesirable type of noise suppressor network is one which possesses atrigger action so that any stable value intermediate the normal andcut-off bias values is eliminated. A noise suppressor arrangementpossessing such a trigger action is shown in Fig. l. This noisesuppressor circuit generally comprises an oscillator whose operation isunder the control of the incoming signals, and whose oscillatory outputis rectified and utilized for the muting action on the audio amplifierof the receiver. The suppressor arrangement is constructed so as toinvolve a minimum of circuit elements, and comprises tube 2| of the 6B7type whose pentode section and two diode sections are arranged as shownin the figure.

I. F. energy is impressed upon one of the diode sections through acircuit which includes the resonant network 22; the latter is tuned tothe operating I. F.; and is magnetically coupled to the tuned circuitl5. The high alternating potential side of circuit 22 is connected tothe diode anode 23, while the low alternatingpotential side of thecircuit is connected to a point on voltage source 24 through a pathwhich includes the resistor R1. The voltage source 24 may be part of thegeneral B voltage supply of the tube- 2|, and for this reason thevoltage source 24 is to be understood as comprising another portion ofthis voltage supply. The cathode of tube 2| is connected to the positiveside of the voltage supply portion 24, while the negative terminal ofthis voltage supply section is grounded. The point on source 24 to whichresistor R1 is connected is negative with respect to the point to whichthe cathode of tube 2| is connected.

The cathodes of tubes 2| and |'u` are at a common direct currentpotential, and are connected by a lead 25. The plate of tube 2| isconnected to the positive terminal of the voltage supply section 24through a resonant network 26, the negative terminal of the voltagesupply section 24 being connected to the cathode of tube 2|. Theresonant network 26 is reactively coupled to the screen grid of thepentode section of tube 2| through an audio frequency transformer 2'|.There is thus provided a local oscillator for producing localoscillations which are to be rectified, and the direct current componentof the rectified oscillations is used to control the audio transmissionefficiency of the receiver.

The oscillations produced by the oscillator just described are impressedupon a rectifier which comprises the diode anode 28. The load resistorR3 is connected to a point on the voltage supply section 24 which isnegative with respect to the point thereon to which the cathode of tubeI6 is connected. The condenser C reactively couples the screen grid ofthe pentode section of tube 2| to the diode anode 28, and comp-rises thepath through which local oscillations are impressed upon the dioderectifier including diode anode 28. The direct current Voltage developedacross resistor R3 is impressed upon the signal input grid` of thepentode section of tube I6 through a path which'includes the resistorR4. and the resistor R5.

'Ihe grid side of resistor Ri is grounded through a condenser C1, andthe anode side of resistor R3 is connected to the opposite side of theresisto-r R4. The direct current voltage developed across resistor R1,by rectification of the I. F. voltage from circuit 22 by the diodeincluding anode 23, is impressed upon the first grid of tube 2|. Theimpression path includes the adjustable tap 3|), the resistor R2 and theresistor Re, the junction of resistors R2 and Re being connected toground through condenser C2.

The operation of the receiving systemwill now be described, attentionbeing directed to Fig. 2 which shows in a graphic manner such operation.Fig. 2 shows the overall selectivity including the combined selectivityof all tuned circuits inclusive of circuit 22. The I. F. voltage ofcircuit I4 is applied to the diode section of tube I6 for demodulation,and the resulting IS, and thereafter amplified and transmitted throughthe subsequent audio frequency network. The pentode section of tube I6functions as an audio amplifier, and the requisite signal grid bias isprovided for the signal input grid of tube I6 through the path whichincludes resistor R5, the resistor R4 and the resistor R3. The cathodeof tube I6 is connected through lead 25 to a point on voltage source 24which is positive with respect to the point on this source to whichresistor R3 is connected. As long as signals above a desired carrieramplitude are being received, the operating grid bias for the pentodesection of tube I6 is the potential difference between the points onvoltage source 24 to which lead 25 and resistor R3 are connected.

When the received signal carrier amplitude decreases below apredetermined intensity level, and the level is such that the AVCnetwork is not functioning to reduce the gain of each controlled tube,background noises will be greatly amplified because the controlled tubesare operating at maximum efliciency. In such a case the backgroundSuppressor arrangement functions to produce an auxiliary, or additional,Ynegative grid bias for the pentode section of tube l5. This isaccomplished by permitting the oscillator section of tube 2| to produceoscillations, and the resulting oscillations are rectified for thepurpose of producing the auxiliary negative grid bias for tube |6. Theoscillator section of tube 2| comprises the platev circuit 26 which isreactively coupled through transformer 2! to the screen grid electrodeof the tube.

The transformer 2'! is a small iron cored transformer having a lownatural frequency between l() and y20 k. c., or a suitable highfrequency as 1600 k. c. Such a high frequency, which is outside thelimits of the operating signal frequency range of a receiver, avoidsundue time delay when starting or stopping the oscillations. As soon asthe amplitude of the signal carrier energy impressed on circuit 22 fallsbelow a predetermined intensity level, the production of localoscillations between the screen grid and plate of the pentode section oftube 2| commences. The 10 k. c. voltage of the oscillator grid coil istransmitted to the diode anode 28 through the coupling' capacity C, andthere is produced a direct current voltage across resistor R3 by viitueof rectification of the locally produced oscillations. Since the signalinput grid of the pentode sectionpof tube I5 is connected to the anodeside of resistor R3, whenever'local oscillations are produced Vtherewill be impressed upon the signal grid of tube I6 the auxiliary negativebias referred to heretofore.

This auxiliary bias is ,sulcient in'magnitude to substantially preventthe amplification of audio signals in the pentode section of tube I6.'I'he circuit elements R4 and C1 function as a 'llternetwork to suppressthe fluctuating component of the rectified current ilowingthroughresistor R3 from reaching the signal gridfof tube I6. When the receivedcarrier amplitude rises above the predetermined intensity level the I.F. voltage on the loosely coupled, sharply tuned circuit 22 rises nearresonance. This voltage is rectied in the diode rectifier circuit'whichincludes the diode anode 23 and resistor R1, and there is produced adirect current voltage across the load resistor R1. The grid nearest thecathode of tube 2| is connected to a desired negative point on resistorR1 through the path which includes resistors Rs and R2 and theadjustable tap 30. Therefore, it will be seen that a part of the directcurrent voltage developed across resistor R1 is applied through resistorR6 to the rst gridfof tube 2|, and the magnitude of this applied voltageis sufficient to cut olf the plate current flow through the pentodesection of tube 2|. This results in a cessation of local oscillations;and as a consequence the rectified 'voltage across resistor R3disappears. VThis removes the auxiliary cut-01T bias from the audioamplier pentode section of tube I6, and thus permits the normaltransmission of audio signals from the demodulator to the reproducer ofthe system.

The primary or energizing circuit I4 is coupled Y at optimum coupling tothe secondary circuit I5 which is used to feed the demodulator diode.This secondary circuit is also loosely coupled to the tertiary circuit22 which is not coupled directly to I4. This arrangement is important asthen the AVC (which is derived from I3) is actuated by a circuit passingay wider frequency band than is passed by the additional selectivity ofcircuit 22 which energizes the squelch diode 23. This is important forthe following reasons: If tuned to a carrier frequency, the circuit 22will build up an I. Fand diode load voltage of a certain magnitude ofcertain ratio to the voltage in I5. With this ratio the squelch diodeoperates and unlocks the audio system. If thev circuits are, however,not tuned to a carrienthe wider noise spectrum of circuit I3 produces anAVC voltage which in relation to the squelch diode voltage produced by Ynoise in the more selective circuit 22 is now considerably greater, andas the ratio is greater, Ythe squelch circuit will not unlock the audiosystem;

provided that kthe AVC has adjusted the voltage in I 5 to a magnitudecomparable with the carrier magnitude assumed before, which is the casefor sets with good AVC. This arrangement thus discriminates betweenrelatively large noise levels and fairly weak carrier signals. The pointbeing: eXtra selectivity on the squelch diode circuit. A

second point is that the time constant of the AVC must be shorter thanfor the squelch, so that the AVC has decreased the noise voltage in I5before diode 23 had time to produce sufficient bias on R1 to stop theoscillator 2|.

The resistor Re is an alternating current load resistance on the controlgrid of tube 2| at 10 k. c. Its function is to increase the differenceof the values e1 and e2, see Fig. 2, as the oscillating elements areboth positive, and thus does not produce any self bias rfor theoscillator system. As the control grid of tube 2|` is loaded atoscillator frequency by resistor Raam alternating current voltage isgenerated on resistor Re due to coupling in the tube 2|. nating currentvoltage cause grid current to ow through the control grid of tube 2 I,and this grid.

current in turn causes a self bias on the control griddue to resistorsR2, R1 and condenser Cz. There is secured in this fashion for theoscillator circuit an overlapping start and stop characteristic. Therequired direct current control voltage on the control grid of tube 2|is very small as in practice the second grid, or oscillator grid, isonly about 10 volts positive. This makes the circuit sensitive to smallchanges in carrier voltage which is essential forV receivers withsatisfactory AVC action. Y Y v The oscillator including tube 2| may beeasily adjusted to start at a certain negative grid bias value, i. e., adefinite transconductance for small amplitudes, but to continueoscillating if once started, with considerably higher bias values (selfbias on resistors R2 and the portion of R1 below slider 30) as thetransconductance for larger amplitudes remains higher as the startingvalue for small amplitudes at the Vsame grid bias voltage. Thisactioncauses theoscillator associated with the plate and screen grid of tube2| to stoposcillating if the rectified I. F. signal voltage developedacross resistor R1 increases to a predetermined value, e1, (see Fig. 2)but the voltage across resistor R1 must be decreased to a considerablylower value e2 before oscillations will be produced again. The voltagee1 may be adjusted by means of slider 3Q so as to be reached near, c-rat, resonance when the I. F. amplitude rises due to Vtuning in a signal.Y

However, if the signal is tuned in, a slighty detuning or fading of thesignalwill not start'the oscillator action because the value e2 isconsiderably lower. This is clearly shown in Fig. 2 where- The positivepeaks of this alter-V in the relationships between the vo1tagese1 and e2and the resonance voltage of circuit 22 are depicted. An impro-per biasvalue on the signal grid of tube I6 is not possible as a stable value,because the minimum'oscillator voltage amplitude to which oscillationsbuild up is greater than the additional bias voltage needed to cut offthe plate current ilow through the pentode section of tube I6.

The tap on R1 determines 'the amount of ,bias` Moved up (in Fig. 1) asmall While the suppressor arrangementzin Fig. 1 is I shown of a highlycompact type, it is to be clearly 75 access-l understood that the diodedemodulator 'may be used to control the production of local oscillationsin the suppressor arrangement, and in that case the oscillator maycomprise a triode wherein the plate and grid are reactively coupled, andthe oscillator may be of the self-bias type. Any common triodeoscillator with grid leak and condenser operates Class C. The grid biasvoltage is highly negative and cuts off the plate current, but the tunedcircuit voltage exceeds the grid bias voltage due to flywheel action ofthe circuit, thus making the grid positive at the peak swing (just likea diode) thus allowing plate current to flow which in turn energizes thetuned circuit. The negative bias is a self bias (diode action) producedby the oscillation voltage, the amplitudes of which are relativelylarge. The operating conditions of the oscillator can be so adjustedthat the transconductance at large swings is larger than at very smallamplitudes. Thus, the oscillator can not start by itself at this biasvalue, but once oscillating, will maintain oscillations. In such a casethe grid of the triode would be controlled in bias from the demodulatorload resistor, and the voltage values e1 and e2 when impressed on theoscillator grid would control the production of oscillations.

Furthermore, the locally produced oscillations in such a case can beimpressed for rectification, for the purpose of producing the oscillatorcut-off bias for the audio tube, upon a diode which is independent ofthe oscillator tube. Furthermore, the frequency of the locally producedoscillations in the noise lsuppressor arrangement may vary over a rangeof values, the essential requisite being that this frequency be outsidethe operating signal frequency range, but be high enough so as to avoidundue time delay when starting or stopping the noise controloscillations.

While I have indicated and described a system for carrying my inventioninto effect, it will be apparent to one skilled in the art that myinvention is by no means limited to the particular organization shownand described, but that many modifications may be made without departingfrom the scope of my invention, as set forth in the appended claims.

What I claim is:

l. In combination with a radio receiver of the automatic gain controltype, a background noise suppressor network comprising an oscillatorresponsive to variations in received signal carrier amplitude, means forrectifying the oscillations to produce a cut-off bias for an audioamplifier of the receiver, and additional means electrically associatedwith the oscillator for imparting a characteristic to the oscillatorsuch that said cutoff bias is produced or removed without anytransitional bias values.

2. In a superheterodyne receiver of the type including an automaticvolume control system adapted to maintain the signal amplitude at thedemodulator substantially uniform over a wide range of signal amplitudevariation at the signal collector, the receiver being characterized byits inclusion of a background noise suppressor arrangement whichcomprises an oscillator adapted to produce local oscillations of afrequency outside the operating signal frequency range of the receiver,means responsive to received signal carrier amplitude variations forcontrolling the amplitude of said oscillations, and additional means forrectifying the oscillations and impressing the direct current voltagecomponent of the rectier oscillations upon a stage subsequent to thereceiver demodulator to render the reproduction of the demodulatedsignals inefficient.

3. In a receiving system provided with a demodulator, a signaltransmission network feeding said demodulator, and means forautomatically varying the transmission eiciency through said network ina sense to maintain the signal carrier amplitude at the demodulatorinput substantially uniform, the receiver being characterized by theinclusion of a suppressor system which comprises an oscillator arrangedto produce local oscillations of a frequency outside the operatinglsignal frequency range of the receiver, means responsive to an increasein carrier amplitude at the demodulator input above a predeterminedintensity level for automatically stopping the production of saidoscillations, and means responsive to said oscillations forautomatically regulating the sound output from said receiver when saidcarrier amplitude decreases below said intensity level.

4. In a receiving system provided with a demodulator, a signaltransmission network feed-l ing said demodulator, and means forautomatically varying the transmission efficiency through said networkin a sense to maintain the signal carrier amplitude at the demodulatorinputsubstantially uniform, the receiver being characterized by theinclusion of a suppressor system which comprises an oscillator arrangedto produce local oscillations of a frequency outside the operatingsignal frequency range of the receiver, means responsive to an increasein carrier amplitude at the demodulator input above a predeterminedintensity level for automatically stopping the production of saidoscillations, and means responsive to said oscillations forautomatically regulating the sound output from said receiver when saidcarrier amplitude decreases below said intensity level, said first meansincluding a device for stopping said oscillations only when said carrieramplitude at the demodulator input decreases to said intensity level.

5. In a receiving system provided with a demodulator, a signaltransmission network feeding said demodulator, and means forautomatically varying the transmission efficiency through said networkin a sense to maintain the signal carrier amplitude at the demodulatorinput substantially uniform, the receiver being characterized by theinclusion of a suppressor system which comprises an oscillator arrangedto produce local oscillations of a frequency outside the operatingsignal frequency range of the receiver, means responsive to an increasein carrier amplitude at the demodulator input above a predeterminedintensity level for automatically stopping the production of saidoscillations, and means responsive to said oscillations forautomatically regulating the sound output from said receiver when saidcarrier amplitude decreases below said intensity level, said rfirstmeans comprising a rectifier having a signal input circuit coupled tothe demodulator input.

6. In the receiving system provided with a demodulator, a signaltransmission network feeding said demodulator, and means forautomatically varying the transmission eiiiciency through said networkin a sense to maintain the signal carrier. amplitude at the demodulatorinput substantially uniform, the receiver being characterized by theinclusion of a suppressorsystem which comprises an oscillator arrangedto produce local oscillations of a frequency outside the operatingsignal frequency range of the receiver, means responsive to an increasein carrier amplitude at the demodulator input above a predeterminedintensity level for automatically stopping the production of saidoscillations, and means responsive to said oscillations forautomatically regulating the sound output from said receiver when saidcarrier amplitude decreases below said intensity level, said last meanscomprising a rectifier re-A actively coupled to said oscillator for theimpression of oscillations on said rectifier, and the output of saidrectifier being impressed vupon an audio amplifier of the receiver.

7. In a receiving system provided with a demodulator, a signaltransmission network feeding said demodulator, and means forautomatically varying the transmission efficiency through said networkin a sense to maintain the signal carrier amplitude at the demodulatorinput substantially uniform, the receiver being characterized by theinclusion of a suppressor system which comprises an oscillator arrangedto produce local oscilla- Vtions of a frequency outside the operatingsignal frequency range of the receiver, means responsive to an increasein carrier amplitude at the demodulator input above a predeterminedintensity level for automatically stopping the production of saidoscillations, and means responsive to said oscillations forautomatically regulating the sound output from said receiver when saidcarrier amplitude decreases below said intensity level, each of said rstand second means including a rectifier, and the electrodes of saidoscillator and two rectiers being disposed within a common tubeenvelope.

8. In a receiving system provided with a demodulator, a signaltransmission network feeding said demodulator, and means forautomatically varying the transmission efficiency through said networkin a sense to maintain the signal carrier amplitude at the demodulatorinput substantially Y uniform, the receiver being characterized by theinclusion of a suppressor system which comprises an oscillator arrangedlto produce local oscillations of a frequency outside the operatingsignal frequency range of the receiver, means responsive to an increasein carrier amplitude at the demodulator input above a predeterminedintensity level for automatically stopping the production of saidoscillations, and means responsive to said oscillations forautomatically regulating the sound output from said receiver when saidcarrier amplitude decreases below said intensity level, said oscillatorincluding an electron discharge tube provided with a pentode section anda pair of diode sections, the plate and screen grid of said pentodesection being reactively coupled for the production of saidoscillations, one of said diode sections being included in said firstmeans, and the other diode section being included in said second meansand having said oscillations impressed thereon through a reactivecoupling from said screen grid.

OTTO H. SCHADE.

